A Primer on Photovoltaic Roof System Installation

August 16, 2018

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Before you jump on to the roof, dive into this overview of solar electric systems for roofs

As the first state in the nation to require all new homes to have solar power beginning in 2020, there’s no doubt that California’s recent decision is causing interest in residential solar systems to soar.

California’s leadership is a giant step forward to bring solar power into the mainstream for all U.S. homeowners. It also promises to cause a major shift in roofing industry for how to adopt methods and materials that are better suited for solar systems.

However, a return on a residential roof-mounted solar investment is only as good as the quality and longevity of the roof where it’s installed. Here’s a quick primer for what to know about solar system roof installation.

Understand the terms

Insolation and irradiance: These are the two different names for quantifying the amount of sunlight reaching an area (usually expressed in the number of sun-hours hitting the solar system per day, month or year based on historical data for the location). The number of sun-hours multiplied by the wattage produced by the system equals the power production in kilowatt-hours. A pyranometer is used to measure solar irradiance or insolation.

Photovoltaic (PV):This term refers to a device that converts sunlight directly into electric energy.

A “cell” is the smallest denominator of a solar collector.

A “modular” are cells arranged in rows and columns and laminated together. A modular has 60 to 72 plus cells wired together (often the size of a large, flat screen TV with a positive and negative lead).

A “string” is a larger denominator of a solar collector consisting of multiple modules. String size is usually limited to about 12 modules.

An “array” is the largest, consisting of one string or hundreds of strings connected.

Crystalline silicone or flexible thin film: These are the two main types of solar modules available. Crystalline can be monocrystalline, polycrystalline or a combo. The flexible thin film is fully-adhered amorphous silicon to a flexible plastic backing but also can be rigid, sandwiched between glass. Because they produce the most power per square foot of surface, 90% of roof-mounted solar panels are crystalline modules. Each adds approximately two to three pounds per square foot of collateral load. Thin films are much lighter and can be adhered to some roof types but require a virgin, clean surface.

Solar ready: A roof that is prepared to be able to add solar panels later if desired. The best time to plan for this is during the design stage.

Installation considerations

The solar community uses “tilt angle” to determine the position of the solar panel while the roofing community uses “pitch.” A lower tilt angle than optimum is not as critical as the panel orientation.

The best design is unobstructed roof area with no shading issues. It’s important to consider plumbing stacks, skylights, chimneys and sidewalls that may create shadows.

Fully ballasted: For flat roofs, ballasted systems require racking to hold solar modules in place. Racking often includes tilt capabilities to orient modules to maximize solar collection. The additional collateral load, depending on design wind speed, is usually 6-10 psf. Roofing manufacturers generally mandate upgrades to the roofing system for this type of application which may include adding sacrificial membranes beneath racking components to prevent membrane abrasion, walk pads for inspection and maintenance traffic and heavier, more durable membrane thickness.

Low Slope: May require a reduced ballast, which involves similar racking or framing components, but can be enhanced by proprietary aerodynamic foils, reducing the amount of needed ballast. Mechanically attached systems are also possible with low-slope and flat roofs and are often used with pre-existing construction.

Solar compatible solar roofing types

The roof is the solar mounting platform. The service life of the roof should always exceed the service life of the solar system. If a roof fails before the system, it can necessitate the decommission, removal and reassembly of the solar system, which can cause serious financial and logistical impact.

Asphalt: Asphalt usually involves stand-off mounts secured into the roof trusses with lag screws. Special flashing kits are used to reduce water leakage around penetrations. Typically, only the highest-quality asphalt roofs can last longer than a solar system, which has an average lifespan of 25 years-plus.

Tile: Like shingles, a sub-flashing is required underneath along with a special top flashing, and mounting hardware. Danger of tile breakage during installation, so cost significantly increases.

Metal: Metal roofs are ideal for solar installation as they are strong, weather-tight, durable and last 50-plus years—longer than solar systems themselves—offering exceptional protection for a solar system investment. They can support framed, rack mounted and thin film laminate (which require a clean and smooth surface, free of imperfections). Solar systems can be mounted on metal without requiring any perforations in the roof, which substantially reduces the possibility of leaks. Installation also is fast and efficient, resulting in further cost savings for materials and labor. While some metal roofs may seem to cost more initially, the long-term aggregated cost is less than a traditional roof system.

For installation, anchor the solar module with the PV direct attachment solution, according to installation instructions and it automatically provides a ground path within the module frames and through the PV direct attachment solution. No lugs or wires are required, except to connect one string of modules to another and ground the system. Stainless steel mounting disks can withstand severe conditions and are compatible with brass mounting clamps for copper roofs. Clamp manufacturers such as S-5! offer tremendous resources for educational resources and training.

Many seam clamps are UL or ETL listed for electrical conductivity and some are also listed for both mounting and electrical bonding. Non-penetrating mounting hardware also is listed to ETL UL 1703 “standard for flat-plate photovoltaic modules and panels.” The mounting disk incorporates nodes designed to ensure module to module conductivity. Non-penetrating seams also greatly improve wind uplift resistance by limiting the possibility of clip disengagement. When placed on the standing seams (between the mounting clip locations), they also limit the possibility of seam separation, the two most common reasons for roof failure caused by wind uplift.

Stone-coated metal roofs: A stone-coated metal roof is made from steel or some other metal; the metal is then coated with stone chips and attached to the steel with an acrylic film. For homeowners who want the style of more traditional roofing materials—yet need a more durable roof to protect their solar roofing system investment—stone-coated offers many of the advantages of standing seam metal roofs.

A solar PV rail system mount bracket that maintains the water-tight integrity of stone-coated metal roofs is essential for proper installation. The key for contractors is to get the training, experience, and qualifications necessary. Stone-coated metal roofing manufacturers, such as Decra and Boral, recommend a variety of training and installation resources from solar rail mount system companies including Quick Mount PV and Solar Roof Hook.

Design planning

With California’s new solar requirements, interest in residential solar systems is predicted to grow significantly. Help your customers get ready now by arming them with questions they need to consider for selecting appropriate roofing materials to make sure their roof is solar-ready.